IL-6 is a pleiotropic cytokine produced by hematopoietic and non-hematopoietic cells, e.g. in response to infection and tissue damage. IL-6 exerts its multiple biological activities through two main signalling pathways, a so-called classic ligand-receptor pathway via membrane-bound IL-6R present mainly on hepatocytes and certain leukocytes, and a trans-signalling pathway via circulating sIL-6R originating from proteolytic cleavage of the membrane-bound IL-6R or from alternative splicing.
In the classic pathway, IL-6 directly binds to membrane-bound IL-6R on the surface of a limited range of cell types. The IL-6/IL-6R complex associates with a pre-formed dimer of the signal-transducing gp130 receptor protein, causing steric changes in the gp130 homodimer and thereby initiating an intracellular signalling cascade. Classic signalling is responsible for acute inflammatory defense mechanisms and crucial physiological IL-6 functions, such as growth and regenerative signals for intestinal epithelial cells.
The extracellular domains of IL-6R and gp130 can be generated without the membrane-anchoring domains by translation of alternatively-spliced mRNAs resulting in sIL-6R and sgp130 variants. Additionally, the extracellular domain of IL-6R can be shed by membrane-bound proteases of the A disintegrin and metalloprotease (ADAM) family (in humans, ADAM17) to generate sIL-6R. In the trans-signalling process, sIL-6R binds to IL-6, forming an agonistic complex which binds to trans-membrane gp130 dimers present on a multitude of cell types that do not express membrane-bound IL-6R; IL-6 signalling by signal transducers and activators of transcription (STATS) is then induced in cells which do not normally respond to IL-6. The activity of the IL-6/sIL-6R complex is normally controlled by high levels of sgp130 present in the circulation which effectively compete with membrane-bound gp130. Trans-signalling is mainly involved in chronic inflammation and has been shown to prevent disease-promoting mucosal T-cell populations from going into apoptosis.
It would be desirable to have a molecule that mimics the natural trans-signalling inhibitor sgp130, but with a higher binding affinity and, consequently, a stronger inhibitory activity. Moreover, it would be desirable to have a molecule that can be administered to humans with minimal toxicity and immunogenic potential.